Alice posts a question. Bob posts an answer. I want to comment on Bob's answer, and I want both Alice and Bob to be notified of the comment. Therefore I type something like "@Alice: As @Bob said....".

I then get a pop-up notice that says I can have only one @-sign per comment. Fine. The same notice tells me that the "owner of the post" will be notified automatically, and therefore I should allocate my @-sign to the non-owner. However, I have no idea whether Alice or Bob counts as the "owner of the post". Is "the post" the question or is it the answer?

Question 1: In this situation, who is the relevant "owner of the post"?

Question 2: Can the pop-up text be modified so that other users won't have to ask this question in the future?

If the post is an answer, the post owner is the author of the answer. In this case, that's Bob. Ideally, you could say something like "@Alice and Bob, (whatever)". If you're commenting under a question, the post owner is the author of the question, and people who've written answers cannot be tagged with @ symbols unless they have left comments under the question (needless to say, they definitely won't be pinged automatically).
I don't think we need a modification in the message because there's an on Meta SE which should be linked in the page the pop-up takes you to: How do comment @replies work? It's probably not feasible to add a direct link to the comment @ replies section because we don't have enough space for that level of detail.